1. Field of the Invention
The present invention relates to quadraphonic sound systems, and more particularly to a system for producing from surround-sound sources two composite signals which when decoded by a suitable decoder reproduce the directional characteristic of the original sound sources.
2. Prior Art
In the CBS Inc. "SQ" system of quadraphonic broadcasting and recording, the various directional sources are picked up with microphones which generally are close to the sources so as to largely contain signals of the individual sources. The outputs of the microphones are connected to an encoder of the type described in Bauer Pat. No. 3,890,466 comprising adders, subtractors and phase shifters, wherein the signals from the directional sources are mixed to produce two composite output signals, LT and RT (Left Total and Right Total), which may be recorded on a two-track medium or broadcast over an FM-stereo transmitter. There are occasions, however, where the sound pickup cannot be carried out conveniently with microphones which are close to the sources. For example, it may be desired to place a group of actors in a semicircle or partial circle in order to perform a dramatic program, and to have them at times walk around. It would be difficult to pick up their movements with ordinary microphones and to encode them properly through a conventional SQ encoder to obtain an appropriate feeling of motion in the decoded signal. As another example, if one desired to broadcast the sounds of an orchestra located in a semicircular or similar arrangement, it would be advantageous to avoid the possible adverse reaction from the audience which might result from the sight of many microphones on the stage.
The basic idea of combining a microphone array with signal processing networks to produce composite signals containing signal information corresponding to a multiplicity of remote acoustical sources suitable for decoding for stereophonic or quadraphonic reproduction is known from British Pat. No. 394,325 of Alan D. Blumlein. The system described in this early patent (1931) utilizes two pressure microphones with associated networks and two velocity microphones arranged at 90 degrees relative to each other to produce a stereo image with two loudspeakers. This patent also combines two velocity microphones in an angular and axial arrangement with suitable networks to produce a quadraphonic image around a motion picture screen.
A modern-day extension of this basic concept is embodied in a system described on pages 222-224 of the Jan. 29, 1976 issue of New Scientist consisting of a microphone using four conventional "cardioid" capsules arranged in a regular tetrahedral array. Each cardioid capsule picks up sound from its front only, and the four capsules point respectively left-back-down, left-front-up, right-front-down and right-back-up. The four "raw" output signals from these capsules are electrically added, subtracted and frequency-equalized, to produce four electrical signals which correspond respectively to the pressure of a sound wave arriving at the microphone and the three orthogonal components of air velocity due to its wave motion. The four derived signals contain height information, but because no one is yet commercially interested in reproducing it, the height information is redundant; thus, one of the four signals is left unused, and the remaining three signals, containing all the azimuth information received by the microphone array are fed to three channels.
The article suggests that for transmission to the domestic consumer, as by disc or tape recording or broadcast, the three (or four) signals would probably be encoded into a conventional pair of stereo signals, or into a stereo pair plus a single extra channel. It is also suggested that various three channel-into-two channel encoding approaches are possible using both phase and amplitude differences between the signals, to produce two channels of encoded sound which may be decoded by a complementary method, generally involving use of sum and difference techniques to recreate the omnidirectional and azimuth channels. However, the article does not described how the encoding is to be accomplished, and specifically states that the hardware cannot be modified to process surround-sound material encoded in the "SQ" format.